Mycobacterium tuberculosis is the cause of tb, and is still a major health problem worldwide. There are a class of proteins that have no known functions, called the PE-PGRS family of proteins. They may be fibronectin binding proteins (fibronectin is one of those things that holds us together at the cellular level), they are involved in disease because if you delete some out Mycobacteria don’t grow as well, and they could be variable surface antigens that allow the bacteria to avoid being seen by our immune system.
Regardless of what it is actually doing, the phages like it. Several of the Mycobacterial phages contain PE-PGRS proteins, suggesting, again, that the phages are helping their hosts cause disease.